Tuning arrangement in radio receivers



June 5, 1945. M E 2,377,475

TUNING ARRANGEMENT IN RADIO RECEIVER Filed May 28, 1945 In venlor .DM, nM BRoSE Attorney Patented June 5, 1945 UNITED STATES PATENT OFFICE-TUNING ARRANGEMENT 'IN RADIO RECEIVERS Dermot Min Ambrose, London,:England, assignor to Standard Telephone and. Cables Limited, London,England, a British company Application May 28, 1943, Serial No. 488,837

In Great Britain June.5, 1942- 5 Claims.

present invention relates to improvements ticularly suitable' for avariety of special applications in electric circuits;

Various different materials are available for the resistance element ofathermistor, these various materials having'different properties-inother respects; as one-- example, a resistance material having a highnegative temperature coefiicient of resistance comprises'a mixture ofmanganese oxide and nickel oxide, with or without the addition ofcertain other metallic oxides, themixture-being suitably heat treated.

Thermistors have been employed in two diiierent'forms: (a) known as aDirectly Heated Therm'is'tor' and comprising a resistance element ofthe-thermally sensitive resistance material pro vided with'suitablelead-out conductors or terminals; and" (b) known as an Indirectly HeatedTherm-istor'comprising the element (a) provided in addition with aheating-coil electrically insulated from'the element. A directly heatedthermistor is primarily intended to be controlled by the current whichflows throgh varies-the temperature andalso the resistance accordingly.Such a thermistor will also be affected by the temperature of .itssurroundings and may'therefore be used for thermostatic control andlike' purposes with or without direct heating by the current flowing,through it. An indirectly heatedthermistor is chiefly designed to'beheated by-acontrolling current which flows through the heating coil andwhich will usually;but not necessarily, be'difl'erent'from'the currentwhich flows th'roughthe resistance element, but this type of thermistor.may also be subjected to either or both of the'types'of controlapplicable to a directly heatedthermistor'.

More-detailed information on the propertiesof' thermistors will be foundin an article by G; L.

Pearson in the Bell Laboratories Record, Dec." l940,'-page"106.

In radio receiving sets, it is now common practice 'to supply some meansof indicating when the set is accurately tuned to a desired transmittingit and which station. For this I purpose cathode ray 1 tube indicatorsof varioustypeshave been: used, and

alsogas-discharge tubes and incandescent electric lamps have-beenemployed where a cheaper arrangementis desired; By'a'ssociating an indirectly heated'thermistor with the circuit, the sensitivity of thesecheaper arrangements may be increased,

According to the invention; therefore, there. is provided a tuningindicating arrangement fora radio receiver provided with automaticvolume control, comprising an indirectly heated ther mistor having itsheating coil connected in series with the plate circuit of a thermionicvalve in the receiver; the plate current of which valve is varied by theoperation of the volume control, the resistance elementof the thermistorbeing connected in a local "circuit with a tuning indicator and adaptedto control the indicator in accord ance with the changes in the tuningadjustmentof the receiver.

The invention will be more clearly understood from the followingdetailed description with reference'to the accompanying'drawing inwhich:

Fig; 1 shows a schematic circuit diagram of one embodiment of theinvention; and

Fign'zshows a modification of part of Fig. 1.

Referring to Fig. l, the valve V is shown as'an intermediate frequency"amplifying valve in a radio receiver, for example. Incoming signals areapplied to the control grid circuit from terminals 1 and'2' and afteramplification are deliveredto the next'stage (not shown) through theoutput mtransformer OT and terminals 3 and 4. The

transformer has its primary winding connected in series with the platecircuit of the valve and the primary and secondary windings are tunedby'mean's of the condensers Cs and Cs respectively.

The cathodeof' the valve is suitably, biased by 7 means ofthe seriesresistanceRz with the usual by- 1 passcondenser Ct and a negativevoltage derived froman automatic volume control arrangement (not shown)of any welhknown type is applied through terminal '5 to an auxiliarygrid of the valve' V." A blocking condenser C1 completes the circuit foralternating currents.

So far the circuit which" has been described'isi provided insa numberconventional; and might be of other well known ways.

In accordancewith the invention; the positive terminal of the hightension supply for the plate of the valve V is connected from terminaLGthrough'the heating coil r of an indirectly heated. thermistor'Twhose-resistanceelement R is connected in parallel with an incandescentelectric lamp L which is lighted from a suitable constant voltage sourceS connected to the terminals I and 8, in series with a constantresistance R1. The source S may supply either alternating or directcurrent, and might be the same as one of the power supplies for thevalve V or other valves of the receiving set.

It will be first assumed that the thermistor has a negative temperaturecoefficient of resist-* ance, so that it has a low resistance when thecurrent through the heating coil r is large.

When the receiver is not tuned to a station,

the incoming signals at terminals I and 2 will be of very low level, andthe automatic volumefcontrol negative voltage at terminal 5 will besmall.

The plate current of the valve V will accordingly be large, thethermistor resistance R will be low, and the lamp L will accordingly beheavily shunted and will glow only faintly, if at all;

When the receiver is brought-into tune thesig'? nal level increases, andthe negative automatic volume control voltage also increases and'reducesthe amplification of the valve Vand also the plate current. Thethermistor .cools and its: resistance R increases; sothat the'lampglows'more bright ly. Accurate tuningwill be: shown by a maxi'-' mumbrilliancy of the lamp, since the plate cur:

rent will increase again as soon as the tunin pointispassed. I I

If the lamp L is connected in series instead of in parallel with theresistance R of-the thermistor T, it; will-be,- evident that theindication of accurate tuning will be given by minimumbrilliancyr.

or the lamp instead of maximum brillianclhand this arrangement may ,bepreferred.

It will be evident that a thermistor with a positive temperaturecoefficient of resistance may also be used. This will give minimumbrightness when the circuit is properly tuned in the arrange-' ment ofFig. 1, and maximum brightness in the series with the resistance R.

Whichever of the four possiblejarrangements is.

employed, it will, of course, be'necessary to choose the voltage of thesources, the resistances 'of the lamp L, and R1, and the range ofresistance change of the thermistor T, so that the-largest variationofthe brightness of the lamp is robe tained corresponding to the-knownvariationsfof" the plate current of the valve V caused-by action of theautomatic volume control; Ihis will not present any difficulty when theconditions are given, and the range of choice of' the thermistorresistance is very wide.

The thermistor T may also be arranged to control a neon tube indicatorinthe manner shown in. Fig. 2. This figure shows the modification of thatpart of Fig. 1 which is below and to-the right-hand side of terminal A;everything above and to the left-hand side of A is the same.

N is a neon tube indicator or otherlike gas discharge device having acathode H or relatively large area, a small control electrode, and aalternative arrangement when the lamp L isyin terminal to the positivehigh tension terminal 6 through a resistance R3. This other terminal ofthe resistance R is also connected through a resistance R4 to thecontrol electrode I! of the neon tube.

Assuming first of all that the thermistor has a negative temperaturecoefiicient of resistance, the resistance R will be a maximum when thereceiver is in tune, the plate current of the valve V-in Fig.1 being aminimum under this condition. This will raise the control electrode l2of the neon tube N (Fig. 2) to a maximum positive potential, and will inturn cause the glow in the tube N to be of maximum brightness and tocover a maximum area on the cathode II. When the receiver is untuned,the control electrode I2 will be at a low potential and there will belittle or no glow.

A thermistor with a positive temperature 00- efficient of resistancewill produce a substantially similar'effect if R and R3 are interchangedin Fig. 2. In this case the resistance R will be a minimum when thereceiver is in tune so that it will cause the control electrode l2 toassume a maximum positive potential as before.

It will generally be preferable to operate the neon-tube from'the samehigh tension supply as the valve-V as indicated in Figs. 1;and'2;;but'ifpreferred, a separate supply could be used, .which would be connected toterminal ill, the connece I tion Bbetween terminals 6 and ill beingremoved.

It is of course not essential to apply the invention to an intermediatefrequency valve. 1 [Any other valve similarly controlled so that itsplate current is varied in accordance with level'could be used instead;1

By suitable choice of the resistance 'r of the heating coil of thethermistor T in Figs. 1 and 2, the sensitivity of the indicators can bemade the signal greaterthan if the thermistor were not used. 1 I

What isclaimed is: i i

1. In a radio receiver having an amplifying valve including a platecircuitthe-plate current of which valve is variedbythe level of signalsentering said vane; a tuning indicating devicecom, prising. a thermistorhaving a thermally sensitive resistance element and a heatingcoil;electrically insulated .from'. said element, a series connectionfrom s'aidcoil to said plate circuit a. local circuit connected to saidresistanc element and a tuning indicator in said local circuit connectedto SaidIesistanceeIernent, wherebysaid resistance element is adapted tocontrol said indicator in accordance with the changes in the tuningadjustment of the radio receiver, and whereby no transfer ofelectricalenergy between said plate circuit and said indicator takesplace.

v2. In av radio. receiver having an amplifying valve'including a platecircuit the, plate current of which valve is varied by,,the level ofsignals reaching said valve, a tuning indicating device comprising athermistor having a thermally sensitive, resistance element and cheatingcoil electrically' insulated from said element, a series conmotion fromsaid coil to saidplate circuit, a local circuit. connected to saidresistance element, an

incandescent lampin said local circuit connected to said resistanceelement and a source of electromotiveforcefor supplying current to saidlamp, whereby. {said resistance element controls said incandescent. lampin accordance with the changes in the tuning adjustment of the radioreceiver, and whereby no transfer of electrical energyfbetween saidplate le? tim iece. a H

circuit and said indi 3. In a radio receiver having an amplifying valveincluding a plate circuit the plate current of which valve is varied bysignals entering said valve, a, tuning indicating device comprising athermistor having a thermally sensitive resistance.

element and a heating coil electrically insulated from said element, aseries connection from said coil to said plate circuit, a local circuit,an incandescent lamp in said circuit connected in parallel with saidresistance element, a source of electromotive force for supplyingcurrent to said lamp and a, constant resistance connected in series withsaid source of electromotive force, whereby no transfer of electricalenergy between said plate circuit and said indicator takes place.

4. In a radio receiver having an amplifying valve including a platecircuit the plate current of which valve is varied by signals enteringsaid valve, a, tuning indicating device comprising a thermistor having athermally sensitive resistance element and a heating coil electricallyinsulated from said element, a series connection from said coil to saidplate circuit, a local circuit connected to said resistance element, agas-discharge tube in said local circuit connected to said resistanceelement and a source of electromotive force for supplying current tosaid gas-discharge tube, whereby no transfer of electrical energybetween said plate circuit and said indicator takes place.

5. In a radio receiver having an amplifying valve including a platecircuit the plate current of which valve is varied by signals reachingsaid valve, a tuning indicating device comprising a thermistor having athermally sensitive resistance element and a heating coil electricallyinsulated from said element, a series connection from said coil to saidplate circuit, a local circuit, a gas-discharge tube in said localcircuit, said gas-discharge tube comprising a cathode, an anode and acontrol electrode, a source of constant voltage connected to supply saidtube, a constant resistance across said source and to which saidresistance element is connected in series, a connection from saidcontrol element to the junction point of said resistance element andsaid constant resistance, and a second constant resistance in serieswith said last named connection, whereby no transfer of electricalenergy between said plate circuit and said indicator takes place.

DERMOT MIN AMBROSE.

